1. Field of the Invention
The present invention relates generally to lenses, and more particularly to lenses formed from a plurality of segments some of which have axial refractive index gradients.
2. Prior Art
A conventional lens with spherical surfaces and with a homogeneous index of refraction will not focus light perfectly; there will be spherical and chromatic aberrations. The latter aberrations may be canceled, for example, by using a lens doublet in which each lens has a unique chemical composition and therefore an index of refraction with its own dependence on the wave length of light. The chromatic aberrations can be reduced by cancellation between the two indices. The spherical aberrations can be eliminated by the expensive and difficult process of grinding a predetermined aspherical surface on the lens. It is also well known in the art that these aberrations can be eliminated by employing axial gradient lens blanks. An axial gradient lens is a lens which has an index of refraction profile which varies in one direction only, usually chosen to be the optical axis. These aberration free lenses can be used advantageously in a variety of optical systems, such as slide projectors, cameras, binoculars, and many other imaging devices; the number of lens elements required for a given task can be reduced as well as the weight and complexity of the system.
The blanks for the fabrication of such gradient lenses can be made by a variety of processes such as SOL-GEL, infusion, and diffusion and may be glass, plastic or other suitable optical material. In particular, there is the controlled diffusion process that can produce macro lenses with a prescribable index of refraction axial profile. The fabrication of such axial gradient lenses by the controlled diffusion process is disclosed in U.S. Pat. No. 5,262,896, "Refractive Elements With Graded Properties and Methods for Making Same", to R. Blankenbecler, which patent is incorporated herein by reference. These lenses are available from LightPath Technologies of Tucson, Ariz.
The above discussion applies to both radial and cylindrical lenses; however the grinding and polishing of cylindrical lenses to the needed precision is especially difficult. Cylindrical lenses condense or expand a beam of light in one transverse dimension only; they can focus light into a thin line and are used in laser scanners, fax machines, laser printers, and in the Cinemascope process, for example. Cylindrical lenses also can be used to produce a symmetrical output beam from an unsymmetrical source such as a laser diode. An improved type of cylindrical lens that does not require a cylindrical surface on the lens, yet offers the optical designer increased flexibility, is highly desirable.
A lens design for coupling a laser diode to a multimedia fiber using anamorphic radial gradient-index components is described by J. M. Stagaman and D. T. Moore, "Laser diode to fiber coupling using anamorphic gradient-index lenses", Applied Optics, vol. 23, no. 11, pp. 1730-1734 (1984). These authors discuss the disadvantages and difficulties in the conventional approach of utilizing prisms and/or cylindrical lenses. Their optimum design for a lens system to be used with a laser diode source with astigmatism uses a gradient index lens with an elliptical profile. However, there is no known method to fabricate a general anamorphic lens in which each transverse dimension has its own independent index profile. A new type of lens that allows the fabrication of a general anamorphic lens is highly desirable.
A monolithic anamorphic lens having at least one curved surface and an axial gradient index parallel to an optical axis is disclosed in U.S. Pat. No. 4,900,138 to Atkinson, III et al., issued Feb. 13, 1990. This patent also describes other gradient index profiles and is incorporated herein by reference.
A cemented lens design in which two (or more) different types of homogeneous glass are ground into the proper shape then polished and cemented together, is well known in the art. In Applied Optics, by Leo Levi, Vol. 1, John Wiley & Sons, New York, (1968), it is stated that with a proper choice of glasses and shapes, it is possible to reduce both chromatic and spherical aberrations in a cemented lens despite the severe restrictions on available choices. A variation of this type of lens has also been described by A. C. S. van Heel in "One-Radius Doublets", Optica Acta, Vol. 2, pp. 29-35 (1955).
Also known to the art is a segmented lens design in which two different types of homogeneous glass with different values of the index of refraction are bonded together by heat and the interface molded (slumped) into a prescribed shape. The external faces are then ground flat. This is a monolithic lens with a discontinuous index of refraction across a smooth aspherical interface. This lens has low optical power and has been proposed as a corrector plate. Such a lens is disclosed in U.S. Pat. No. 2,596,799, "Aberration Corrector Member for Image Forming Optical Objectives", to Tillyer, et. al., issued May 13, 1952.
Another type of segmented lens known to the art is built up of constituents of different indices of refraction as described by W. Ewald, in "Lens for Optical Purposes" U.S. Pat. No. 1,943,521, issued Jan. 16, 1934. The separate parts of the lenses, each of which is homogeneous, are cemented together in such a manner that the boundary surfaces or interfaces are substantially located in the direction of the path of light rays. That is, the interfaces are parallel to the optical axis. The indices of refraction are chosen so as to reduce the spherical aberration of the lenses and produce clearly defined images on a screen.
A patent teaching the fabrication and design of a double axial gradient lens blank has been granted to the present inventor, R. Blankenbecler, "Double Axial Gradient Lens and Process for Fabrication Thereof", U.S. Pat. No. 5,044,737, issued Sep. 3, 1991. A diffusion process produces a monolithic lens with a continuous index of refraction profile; the lens is composed of three regions, front, center, and rear, each of which can have its own graded index of refraction profile and chemical composition.
A patent teaching the forming of a cylindrical or spherical gradient lens blank from an axial gradient lens blank by heat molding (slumping) has been granted to R. Blankenbecler and M. Wickson, "Shaped Gradient Fabrication In Lenses By Molding From Axial Gradient", U.S. Pat. No. 5,236,486, issued Aug. 17, 1993. This process produces a monolithic lens with a continuous index of refraction profile.
A design for a cemented gradient index lens system for laser beam reshaping is disclosed by C. Wang and D. L. Shealy, "Design of gradient-index lens systems for laser beam reshaping", Applied Optics, vol. 32, pp. 4763-4769 (1993). A system using two axial gradient lenses and a homogeneous central transfer lens is disclosed. The front and rear faces are flat planes. The interfaces between the front gradient lens and the central transfer lens and the central transfer lens and the rear gradient lens are spherical surfaces that must be ground and polished to fit into each other. In addition, the gradient index profiles are different and must be chosen properly to function as a beam reshaper. A simpler design that does not require multiple spherical surfaces to be ground and polished to such precision is highly desirable.
As mentioned above, spherical and chromatic aberrations will be present in lenses with spherical or cylindrical external surfaces. A suitable index of refraction profile (essentially linear) in the spherical or cylindrical lens cap can be used to cancel the spherical aberration and form a precise image. However this normally requires a large change in index across the profile. Furthermore, the surface of the cap must be ground and polished in a region of varying index, varying hardness, and varying coefficient of thermal expansion which is a difficult process to carry out with accuracy. An aspherical shaped surface on a homogeneous lens will also reduce this particular aberration, but the fabrication of such a lens surface with the required accuracy is a very difficult process. Even the grinding and polishing of a simple cylindrical surface on a lens blank is difficult and expensive. Thus, a new cylindrical lens fabrication technique that eliminates this fabrication step by replacing the curved surface with a plane surface yet eliminates spherical aberration is highly desirable.
It is therefore an object of the invention to provide an improved type of lens having the function of a cylindrical lens that does not require a cylindrical surface on the lens.
It is another object of the invention to provide a general anamorphic lens in which each transverse dimension has its own independent index profile.
It is a further object of the invention to provide a cemented gradient index lens system for laser beam reshaping that does not require multiple spherical surfaces to be ground and polished to a high precision.
It is still another object of the invention to provide a lens system which provides independent manipulation of a beam of light in two transverse orthogonal directions.